The U.S. Environmental Protection Agency rates coal ash ponds according to a National Inventory of Dams (NID) criteria that categorizes the ponds by the damage that would occur in the event of a dam failure. Coal ash dams are usually built from a combination of soil and ash and often impound millions of tons of toxic coal ash and wastewater. The majority are over 40 years old, and most do not have monitoring to detect leaks of toxic pollutants.
There are 331 High and Significant hazard coal ash ponds in the United States. The NID hazard potential ratings refer to the potential for loss of life or damage if there is a dam failure:
High Hazard (81 ponds)
Failure or mis-operation of these dams will probably cause loss of human life. (Designations of * are based on state determinations. EPA considers the hazard potential of these dams to be significant.)
Significant Hazard (250 ponds):
Failure or mis-operation of these dams results in no probable loss of human life, but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns.